In chapter six of the Life of Constantine-Cyril, the author speaks of an invitation being sent by the Arabs to Constantinople to hold a religious disputation, with particular reference to the Holy Trinity and belief in one God. The emperor thereupon is said to have convoked the Senate in order to deliberate as to what should be done. He asked Constantine, known in the text as the Philosopher, at that time only twenty-four years old, to go to the Arabs, probably in Samarra near Baghdad which was the residence of the Caliph, and to take part in the religious discussion. The year this reportedly took place was in 851.
Life of Constantine
(Ch. 6)
Afterward the Hagarites, who were called Saracens, blasphemed the single Deity of the Holy Trinity, saying: “How is it, O Christians, that you, while holding that God is one, further divide Him into three, saying He is Father, Son, and Spirit? If you can explain clearly, send us men who can speak of this and convince us.”